塑鉸區箍筋形式對混凝土大梁耐震行為之影響

Abstract

鋼筋混凝土韌性抗彎矩構架中，由於大梁塑鉸區具高剪力與圍束需求，須配置重疊閉合箍筋，若按設計規範所示之箍筋進行配置將有一定施工難度。本研究共配置五座不同形式閉合箍筋之大梁試體進行反覆載重試驗，一座配置傳統工法且符合規範之傳統閉合箍筋(S1)，三座配置可提高工作性之閉合箍筋，包含一筆式(S2)、平面搭接式(SL1)與組立搭接式(SL2)閉合箍筋，一座配置實務常見但不符規範之2大U閉合箍筋(S3)。結果顯示，S2、SL1與SL2試體之內箍筋有效束制斷面中央3根主筋，並於位移比6%時，挫屈區域發生主筋或內箍筋斷裂情形，使鋼筋充足發揮消能性。相對於S1試體，S2、SL1與SL2試體之極限位移比∆u高約1%、8%與13%。並於位移比4%時，S2、SL1與SL2試體之等效阻尼比ξeq高約7%、13%與8%，於位移比5%時，ξeq高約39%、35%與31%。此外SL1與SL2試體之搭接內箍筋仍有效束制斷面中央主筋，且能發展至降伏應變，顯示即使塑鉸區混凝土已發生開裂剝落，混凝土核心仍有效提供箍筋搭接區域足夠握裹長度，確保試體為具韌性撓曲破壞。S3試體因僅大U箍筋束制角隅主筋且圍束混凝土能力較差，相對於S1試體，S3試體之∆u低約15%。並於位移比4%時，S3試體之ξeq低約2%，且頂蓋側主筋受壓時強度陡降而未通過ACI 374.1-05耐震性能評估。

In special moment resisting frames, hoops should be provided within the plastic hinge regions for the high shear and confinement demand. However, conventional hoop configurations conforming to the code requirements are difficult to be constructed. In this research, a total of five beam specimens with different types of hoops were designed and tested under cyclic loading. One specimen was designed with hoops (S1) with a conventional configuration conforming to the design code for reinforced concrete. Three specimens were designed with innovative hoops that can improve constructability. The innovative hoops included continuous hoops (specimen S2), two-dimensional lap-spliced hoops (specimen SL1), and three-dimensional lap-spliced hoops (specimen SL2). One specimen was designed with two overlapping perimeter hoops (S3), which are common in Taiwanese practice but do not meet the code requirement for hoops in the plastic hinge regions of beams. Test results showed that the inner hoops of specimens S2, SL1, and SL2 effectively restrained the three main bars confined by the inner hoops. When the drift ratio of the beam reached 6%, the main bars or the inner hoops fractured in the buckling region. This means the steel bars were fully mobilized to resist the load and dissipate energy. Compared with specimen S1, the ultimate drift ratio (∆u) of specimens S2, SL1 and SL2 was higher by 1%, 8% and 13%, respectively. Moreover, when the drift ratio of the beam reached 4%, the equivalent damping ratio (ξeq) of S2, SL1, and SL2 was higher by 7%, 13%, and 8%, respectively. When the drift ratio of the beam reached 5%, the ξeq of S2, SL1, and SL2 was higher by 39%, 35%, and 31%, respectively. The main bars of specimens SL1 and SL2 were still effectively restrained, and lap-spliced hoops of specimens SL1 and SL2 developed into the yield strain, which showed that even if the concrete in the plastic hinge region had cracked and spalled, the concrete core still effectively provided sufficient confinement to the lap splices of the hoops to ensure that the specimens had a ductile flexural failure. Compared with specimen S1, the ∆u of specimen S3 was lower by 15%. Moreover, when the drift ratio of the beam reached 4%, the ξeq of S3 was lower by 6%. When the main bars confined by the cross-tie cap were subjected to high compression, the strength of the beam dropped rapidly. The specimen failed to pass the seismic performance evaluation of ACI 374.1-05.